A population-based case-control study of male breast cancer is proposed to: a) elucidate risk factors for breast cancer other than those related to menstruation and childbearing; and b) provide additional information on the role of hormonal factors in the genesis of breast cancer. Eligible cases will be all adult males who develop breast cancer from 1 July, 1983 to 30 June, 1986, and who reside in areas covered by 9 population-based cancer registries in the United States funded by the Surveillance, Epidemiology and End Results (SEER) program of the National Cancer Institute. A random digit dialing technique will be utilized to select two controls per case from among residents of the same counties in which the cases reside. All study subjects will be interviewed to ascertain: a history of a variety of factors related to endogenous sex hormone levels, use of exogenous sex and other hormones, use of drugs that may cause gynecomastia, family history of breast diseases and cancer, exposure to ionizing radiation, selected dietary factors, and history of prior neoplasms. Medical records of the cases will be reviewed to ascertain evidence of Klinefelter's syndrome and concurrent gynecomastia, and slides from all cases will be reviewed and classified by a single pathologist. Data processing and analysis will be performed at a Coordinating Center in Seattle. Both stratification and multivariate techniques will be used to estimate relative risks of breast cancer in relation to the various hormonal and non-hormonal variables considered.